Power inverters have long been used to convert D.C. power to A.C. power. Generally, such inverters are used where A.C. powered devices, such as televisions, electric drills, hair dryers, and the like, are to be used, but A.C. power is not available. For example, inverters may be used to power such A.C. powered devices on boats or mobile homes, or in residences in remote locations where A.C. power lines have not been routed. In these locations, D.C. power will be available from batteries which are periodically charged, either from A.C. power as it is available or from wind, solar, or water generator sources. Battery-powered inverters may also be used to provide non-interruptable A.C. power in the event of an interruption is service from commercial power suppliers.
Early power inverters were of the electromechanical variety in which a D.C. power motor was simply coupled to an A.C. generator (alternator), with some regulation of rotational speed being provided to control frequency. Voltage regulation was then provided by controlling the alternator field current. These electromechanical power inverters were invariably large, heavy, noisy, inefficient, unreliable, and expensive.
The advent of solid-state switches, such as high current, low impedance, field effect transistors (FET's), has made practical the design of solid-state inverters. These solid-state inverters generally switched the D.C. power source to the primary winding of a step-up transformer with alternating polarities, thereby generating a higher voltage A.C. signal having the same frequency as the switching frequency. Solid-state inverters are relatively efficient in converting D.C. power to A.C. power because little power is dissipated in the switching devices. When the switching devices are open, the currents through the devices are virtually zero, so that the devices dissipate virtually no power in their "off" state. When the switching devices are closed, the voltage drops across the devices are very low, so that the devices dissipate very little power in their "on" state. The switching devices are switched rapidly between their "on" and "off" states so that they are virtually never in a linear state in which power dissipation would be substantial.
Inverter efficiency is important under both "load" and "no load" conditions. Efficiency at high loads is important because significant power is most likely to be dissipated when the inverter is outputting substantial power. For example, an inverter that is 95% efficient dissipates 100 watts when putting out 2000 watts, while only 1 watt when putting out 20 watts. Efficiency when power is not being drawn from the inverter is also important because inverters often must be continuously powered so that they can supply A.C. power on demand. Thus, although it is relatively easy to make the inverter efficient at no load, even small inefficiencies can be significant because inverters often spend most of their time in an unloaded condition.
Power inverters generally do not actively regulate their output voltages. Instead, they attempt to provide a low output impedance so that the output voltage will not drop significantly until the load impedance begins to approach the output impedance of the inverter. While this approach is satisfactory under no load conditions and even for some high load applications, it is unsatisfactory under high load conditions for many applications or where a precise A.C. voltage is required. Also, without active voltage regulation, the magnitude of the A.C. voltage will vary as a function of D.C. (battery) voltage as well as load impedance. Thus, the A.C. voltage will gradually drop as a battery supplying the D.C. power is gradually discharged.
Two approaches to voltage regulation would be to vary either the duty cycle that the D.C. power is switched to the transformer or the magnitude of the D.C. current flowing through the primary of the transformer. Each of these approaches has serious disadvantages and limitations. For example, controlling the magnitude of the D.C. current flowing through the primary of the transformer generally requires that solid-state circuits operate in their linear region, thus dissipating substantial power. As a result, conventional inverters have not provided a relatively inexpensive and efficient means of generating A.C. power having a precisely regulated voltage.
Most conventional power inverters do not generate a waveform that replicates a sine wave of the type provided by electric utilities. Instead, inverters generally output a 60 Hz square wave with leading and trailing edges corresponding to the times that the D.C. power is switched to the inverter's transformer. While many A.C. appliances are capable of satisfactory operation when powered by a square wave, some must be powered by a closer approximation of a sine wave. Also, the high-frequency spectral components in a 60 Hz square wave applied to an A.C. appliance can cause interference in radio receivers used in such devices as televisions, communications radios, cellular telephones, and the like.
Some more sophisticated inverters attempt to more closely replicate a sine wave by generating a "modified sine wave," which is simply a square wave having a dead band between positive and negative cycles. Inverters generating a modified sine wave having a dead band are able to provide some voltage regulation by varying the duty cycle of the dead band. However, these modified sine waves still contain significant high frequency spectral components which can also cause radio frequency interference.
As mentioned above, power inverters often receive D.C. power from batteries. These batteries are sometimes charged by a battery charger from A.C. power intermittently supplied by an electric utility or an on-board, motor-driven alternator (genset). Some conventional power inverters are adapted to also function as a battery charger. In these combination inverter-chargers, A.C. power is supplied to the secondary winding of the inverter's transformer, and either the switches are operated in synchronism with the A.C. power, or diodes are provided to rectify the lower voltage A.C. power on the secondary of the transformer.